The present invention relates generally to an automatic transmission, and more particularly to a process of escaping from an interlock state due to a failure in which an engaging element is incorrectly applied.
There is a vehicle-automatic transmission including a control system that detects electrical failures in solenoids and sensors provided in a hydraulic control circuit, and performs a shift control operation in accordance with the state of the failure.
In addition to such electrical failures, mechanical failures, such as valve stiction and entrance of foreign matter, are possible in each valve for regulating a hydraulic pressure supplied to a frictional engaging element.
It is possible that, due to such valve failures, an engaging element to be disengaged is incorrectly engaged to cause an interlock state of the automatic transmission.
Japanese Patent Application Publication No. 2003-49937, henceforth referred to as “JP2003-49937”, shows a process of escaping from an undesired interlock state of an automatic transmission. In order to detect an abnormality due to stiction of a valve for regulating a hydraulic pressure supplied to each frictional engaging element, the automatic transmission includes a signal-pressure-operated valve which operates in response to occurrence of at least an abnormality in the hydraulic pressure supplied to one of the frictional engaging elements. The automatic transmission also includes a hydraulic switch provided in downstream of the signal-pressure-operated valve to detect a change in the hydraulic pressure due to operation of the signal-pressure-operated valve. When the hydraulic switch detects a change in the hydraulic pressure, the hydraulic pressure of each engaging element is relieved in order to bring the automatic transmission into a neutral state and thereby to escape from the interlock state.